In thermally assisted magnetic recording wherein a magnetic recording medium is irradiated with an evanescent light whereby the surface of the medium is locally heated and the coercive force of the medium is decreased, and writing is effected. This thermally assisted magnetic recording attracts attention as a magnetic recording system of the next generation which is capable of realizing a high plane recording density of approximately 1 T bit/inch2 or larger. In the case when the thermally assisted magnetic recording system is adopted, even when a recording medium having a coercive force of several tens kOe at room temperature is used, writing can easily be effected with a magnetic head having the currently available magnetic recording field.
Therefore a magnetic material exhibiting a high magneto crystalline anisotropy Ku of higher than 106 J/m3 can be adopted for the recording layer. Thus, average particle diameter of magnetic crystal grains can be reduced to 6 nm or smaller while a high thermal stability is maintained. As such high Ku material, there can be mentioned, for example, a FePt alloy with an L10 type crystalline structure having a Ku of 7×106 J/m3 and a CoPt alloy having a Ku of 5×106 J/m3.
In the case when a FePt alloy with an L10 type crystalline structure is used for the magnetic layer, the FePt alloy crystal grains must be (001)-ordered. It is preferable that this magnetic FePt alloy layer with an L10 type crystalline structure is formed on a (100)-ordered MgO-containing underlayer. The (100) plane of MgO exhibits good lattice constant conformity with the (001) plane of L10 type FePt alloy. Therefore, when the magnetic FePt alloy layer with an L10 type crystalline structure is formed on the (100)-ordered MgO-containing underlayer, the resulting magnetic layer exhibits (001) orientation.
To decrease a media noise and enhance an SR ratio of the magnetic recording medium, the particle diameters of magnetic crystal grains must be rendered fine even in the thermally assisted magnetic recording medium. For this purpose of rendering fine the magnetic crystal grains, it is effective to incorporate an oxide such as SiO2 or TiO2 as a grain boundary segregation material in the magnetic layer. That is, FePt crystal grains can be of a granular structure such that the crystal grains are surrounded by the added oxide such as SiO2.
The particle diameters of magnetic crystal grains can be rendered fine by adding an increased amount of the grain boundary segregation material. For example, it is described in J. Appl. Phys. 104, 023904, 2008 that the particle diameters of FePt alloy magnetic crystal grains can be reduced to 5 nm by the addition of 20 volume % of TiO2. Further, it is described in IEEE. Trans. Magn., vol. 45, 839, 2008 that the particle diameters of FePt alloy magnetic crystal grains can be reduced to 2.9 nm by the addition of 50 volume % of SiO2.
The magnetic layer of a thermally assisted magnetic recording medium is preferably formed from, for example, a FePt alloy with an L10 structure having a high Ku. To reduce the media noise of the thermally assisted magnetic recording medium, crystal grains of the FePt alloy must be rendered fine. Therefore, a grain boundary segregation material such as an oxide including SiO2 or TiO2, or carbon is preferably added into the magnetic layer. It is to be noted that the addition of a grain boundary segregation material is effective for rendering the crystal grains fine, but, the dispersion of particle diameters is generally difficult to narrow.
As mentioned above, the magnetic layer comprised of a FePt alloy with an L10 structure is preferably formed on a MgO-containing underlayer. In the case when the crystal grains in the MgO-containing underlayer have a large particle size, plural crystal grains of FePt alloy grow on one particle of the MgO crystal, and thus, the particle diameters of the grown FePt alloy crystal grains are not uniform and the dispersion of particle diameters is large. To reduce the media noise, it is essential to render uniform the particle diameters of magnetic crystal grains, and therefore, it becomes essential to render fine the particle diameters of the MgO-containing underlayer, as well as rendering fine the particle diameters of magnetic crystal grains.